1. Field of the Invention
This invention relates to a process for the synthesis of hard boron nitride having a very large hardness as well as high heat resistance. The hard boron nitride of the present invention can effectively be applied to the field needing excellent properties such as hardness, heat resistance, oxidation resistance, chemical stability, etc., e.g. cutting tools, wear resistance tools and the like.
2. Description of the Prior Art
The so-called hard boron nitrides such as cubic boron nitride and wurtzite boron nitride can be said to be ideal as a material for tools such as cutting tools, wear resistance tools, etc. because of having a hardness next to diamond and being excellent in heat resistance, oxidation resistance and chemical stability in comparison with diamond. However, since a superhigh pressure and high temperature are required for the synthesis of hard boron nitride, than in the case of diamond, the shape of the product is largely restricted and a very expensive superhigh pressure generating apparatus should be used, resulting in a high synthesis cost and a limitation on the range of use thereof.
As in the case of diamond, processes for the synthesis of hard boron nitride have lately been developed comprising depositing on the surface of a substrate from gaseous phase without using a superhigh pressure and high temperature. According to this gaseous phase synthesis technique, it is considered that the shape of the product is hardly limited and the synthesis cost is largely decreased because of using no superhigh pressure generating apparatus.
As the gaseous phase synthesis of hard boron nitride, there are proposed a number of methods, for example,1 an ion beam method comprising melting and vaporizing metallic boron by an HCD electron gun, reacting with nitrogen plasma in the ambient atmosphere and thus depositing and synthesizing boron nitride on the surface of a substrate, 2 a reactive sputting method comprising sputtering metallic boron with hydrogen or nitrogen, reacting with nitrogen plasma in the ambient atmosphere and thus depositing and synthesizing on the surface of a substrate and 3 a plasma CVD method comprising reacting diborane with nitrogen or ammonia in an atmosphere of high frequency such as microwave and depositing and synthesizing boron nitride on the surface of a substrate.
However, all the above described prior art methods have not succeeded in depositing and synthesizing cubic boron nitride or wurtzite boron nitride of single phase on a substrate from gaseous phase, but have obtained only a mixture of hard boron nitride of cubic or wurtzite type and hexagonal boron nitride of a thermodynamically low pressure phase. The hexagonal boron nitride is not suitable for a tool material because of its low hardness and accordingly, the gaseous phase synthesis boron nitride according to the prior art method has not been put to practical use as a tool material.
In view of the above conditions the present invention has been made for the purpose of providing a novel gaseous phase synthesis process for obtaining hard boron nitride on the surface of a substrate as a substantially single phase, which can practically be used as a tool material.